Organic Chemistry II: Comprehensive Study Notes (University of Ghana)

Introduction to Organic Chemistry

  • Organic chemistry is the scientific study of carbon compounds and the reactions they undergo. Exceptions: some inorganic carbon-containing species are not organic (e.g., Na₂CO₃, KCN, CO₂).

  • There are over 16 million known carbon-containing compounds.

Carbon, Catenation, and Geometries

  • Group 4 elements form chains with themselves (catenation). The ability to catenate decreases down the group.

  • Carbon is unique: electronic configuration 1s^22s^22p^2; it has room for 4 bonds to fuse with 4 other atoms.

  • Organic compounds exhibit defined geometries around carbon–carbon bonds:

    • Four attached substituents → tetrahedral geometry with bond angles heta \,=\, 109.5^ ext{\circ}

    • Three attached substituents → trigonal planar geometry with bond angle heta \,=\, 120^ ext{\circ}

    • Two attached substituents → linear geometry with bond angle heta \,=\, 180^ ext{\circ}

Common Geometries and Examples

  • Tetrahedral: Methane ext{CH}_4

  • Trigonal planar: Ethene ext{C}2 ext{H}4

  • Linear: Ethyne (acetylene) ext{C}2 ext{H}2

Macromolecules and Life (Biomolecules)

  • Living organisms rely on macromolecules: large organic compounds polymerized from smaller units.

  • Three main types of macromolecules in living organisms:

    • Proteins

    • Carbohydrates

    • Nucleic acids

  • The basic unit of life is the cell. Cells are composed of macromolecules formed inside the cell, and other macromolecules regulate their formation.

  • Metabolism involves breaking down or building up macromolecules:

    • Breaking down macromolecules releases energy (catabolism).

    • Building up macromolecules requires energy, obtained from catabolic processes.

  • Living organisms must exchange matter and energy with their surroundings, transform matter/energy, respond to changes, grow, reproduce, and protect themselves from stimuli.

  • Formic acid (the simplest organic acid) in red ants, bees, plants, etc.: chemical formula ext{HCOOH}.

  • Ethylene (C₂H₄) acts as a ripening gas; ripe fruit emits ethylene which can ripen other fruits (e.g., sealed bag scenario with tomatoes).

Basic Biopolymers: Proteins, Carbohydrates, Nucleic Acids

  • Proteins: polymers of amino acids; primary structure is the amino-acid sequence (N-terminus to C-terminus). Example representation shows amino acid subunits in a chain.

  • Carbohydrates (saccharides): composed of carbon, hydrogen, and oxygen; monosaccharides (e.g., glucose) and disaccharides (e.g., sucrose) are relatively small sugar units.

  • Nucleic acids: DNA and RNA, built from nucleotides (monomers).

Purity Testing of Organic Compounds

  • Purity tests help determine whether a solid or liquid is pure.

  • Melting Point (MP):

    • A pure solid melts reproducibly over a narrow range (usually < 1^ ext{o}C).

    • Recrystallization increases mp and narrows the mp range by removing impurities.

    • After impurity removal, the mp typically increases.

    • Impure solids may decompose near mp; vacuum (absence of air/oxygen) may be used.

  • Boiling Point (BP):

    • BP is the temperature where vapor pressure equals atmospheric pressure and can help characterize liquids.

    • Impurity effects: bp is less sensitive to impurities than mp; BP decreases with decreasing pressure (≈ 0.5^ ext{o}C per 10 mm decrease in pressure).

    • Boiling chips prevent superheating and ensure smooth boiling.

  • Refractive Index (RI):

    • Refraction occurs when light passes between media with different speeds; defined by the refractive index n, which is wave-dependent and > 1 for most substances.

    • Instrument: Abbe refractometer; uses white light.

  • Visualizing TLC spots: iodine (I₂) forms reversible complexes with many organics; UV-activated fluorescent indicators can be mixed with silica for UV visualization.

Thin Layer Chromatography (TLC)

  • TLC setup: stationary phase = silica gel or alumina; mobile phase = solvent (eluent).

  • Visualization: iodine vapor or UV indicators; adsorption/partitioning yields spots.

  • The retention (Rf) value is defined as:

    • ext{Rf} = rac{d{ ext{spot}}}{d{ ext{solvent}}}

  • Use: monitor reaction progress by sampling at intervals.

  • A pure substance yields a single spot; mixtures yield multiple spots.

  • Preparation considerations: ensure spots remain above solvent front; avoid line-crossing.

Purification Methods (Overview)

  • Pure substance: fixed, sharp mp and bp; melts/boils at a precise temperature.

  • Mixture: physical combination of two or more substances; lacks definite properties; melts/boils over ranges.

  • Key factors when choosing purification methods: nature of the compound, physical state, quantity, impurity level, and stability.

Purification Processes (Major Methods)

  • 1. Distillation

    • Subtypes: Simple distillation, Fractional distillation, Steam distillation, Vacuum distillation.

    • Simple distillation: effective when boiling points differ by > approximately 30–40^ ext{o}C.

    • Fractional distillation: used when bp differences are small (< about 30–40^ ext{o}C); employs a fractionating column for repeated vaporization/condensation; each cycle is a theoretical plate.

    • Vacuum distillation: lowers the boiling point under reduced pressure (useful for heat-sensitive, high-boiling substances).

    • Steam distillation: for heat-sensitive, immiscible with water compounds; volatile together at a temperature lower than either component’s bp (e.g., bromobenzene with water with mixture bp around 95°C).

  • 2. Recrystallization (solids/crystals)

    • Dissolve impure solid at high temperature in an appropriate solvent; crystallize upon cooling; purify by selective solubility.

    • Solvent choice: solvent should dissolve analyte at high temperature but not at room temperature; sometimes solvent mixtures are used.

  • 3. Sublimation

    • Separate compounds that can sublime (solid to gas) without melting; useful for purifying solids that sublime readily (e.g., camphor, iodine, naphthalene, caffeine).

  • 4. Solvent-solvent fractionation (extraction)

  • 5. Chromatography

Distillation Details

  • i) Simple Distillation

    • Best when bp differences are large (> 30–40 °C).

  • ii) Fractional Distillation

    • Used for closely boiling components; involves a column with glass beads to increase surface area for repeated condensation/vaporization; top of column connects to condenser.

    • A thermometer measures the vapor temperature; when a plateau is reached, the liquid with the lowest bp distills first; once it’s exhausted, bp rises to reveal next component.

  • iii) Vacuum Distillation

    • Useful for air-sensitive compounds with high bp; reduced pressure lowers bp; rotary evaporators exploit this principle to recover solvents.

  • iv) Steam Distillation

    • Used for heat-sensitive, water-immiscible compounds; steam lowers the effective bp; setup includes heater, condenser, distiller, separating funnel, etc.

    • Example: a mixture boils below the bp of either pure component (e.g., perfume components).

Purification via Solvent-Solvent Extractions

  • Solid–liquid extraction (Soxhlet): percolator circulates solvent; thimble retains solid; siphon periodically drains solvent; advantage: recycles small solvent amounts to extract large material.

  • Liquid–liquid extraction (Separatory funnel): two immiscible solvents separate; partitioning governed by solubility differences; partition coefficient K = rac{Co}{Cw}; repeated small extractions yield better yields.

  • Practical note: Organic product from an aqueous solution can be extracted into an organic solvent and then evaporated to obtain the product.

Chromatography: Theory and Techniques

  • Chromatography separates complex mixtures based on differential solubility and affinity to stationary vs mobile phases.

  • Key idea: components with polarity more similar to the mobile phase travel faster; those more attracted to the stationary phase move slower; separation relies on polarity differences.

  • Types of chromatography (overview):

    • Column chromatography: separates larger quantities using a packed column with stationary phase; solvent elutes through the column.

    • Gas chromatography (GC): separates volatile mixtures; uses carrier gas (He, N₂); components detected by various detectors; m/z > mass spectrometry is common.

    • High-Performance Liquid Chromatography (HPLC): high-pressure version of liquid chromatography; can handle small particles in the stationary phase to improve separation; requires higher pressures to maintain flow; suitable for sensitive analytes.

    • Thin-layer chromatography (TLC) was already discussed earlier; it is often used for monitoring reactions and quick purity checks.

  • Summary of chromatographic techniques (classification and terminology): mobility depends on stationary/mobile phase; partition/adsorption mechanisms; elution and detection vary by method.

Qualitative Analysis of Organic Compounds

  • Objective: determine which elements are present and their forms.

  • Organic compounds typically contain carbon and hydrogen; qualitative tests identify additional elements such as nitrogen, sulfur, halogens, and oxygen.

  • Sodium fusion test (Lassaigne test): fuse organic sample with molten Na to convert C, H, N, S, halogens into inorganic ions; Na salts (e.g., NaCN, NaCl, NaBr, NaI, Na₂S) are formed; subsequent aqueous mixing and filtration isolate analytes for qualitative tests.

  • Halogens (X₂): reaction with AgNO₃ (or AgNO₃ in aqueous) produces AgX precipitates:

    • AgCl (white) from Cl⁻; AgBr (pale yellow) from Br⁻; AgI (yellow) from I⁻; presence of halide is detected by color and NH₃ solubility patterns.

  • Sulfur: detection via a sulfur test using Pb(CH₃COO)₂ (lead acetate) with basic NaOH filtrate or sodium nitroprusside test; presence indicated by black precipitate PbS or other colorimetric changes.

  • Nitrogen: qualitative test involves fusion to CN⁻ and subsequent reaction with Fe(OH)₃ and ferric chloride to form Prussian blue; or alternative reagents as listed.

  • Oxygen: detected by determining the rest of the mass percentage; oxygen is inferred when not all mass is accounted for by other detected elements.

Quantitative Analysis of Organic Compounds

  • Purpose: determine how much of each element (C, H, O, N, S, halogens) is present in a sample.

  • Methods mentioned include oxidation tests (e.g., HNO₃), combustion (to CO₂ and H₂O), Kjeldahl method for nitrogen, and methods for sulfur and phosphorus speciation.

  • Example outline: CO₂ and H₂O evolution during combustion; mass changes before/after reaction used to compute elemental contents.

Empirical and Molecular Formulas

  • Percent composition: expresses elements as mass percentages in a compound. For a 100 g sample, element masses equal the given percentages.

    • Percent by mass of element X = (mass of X in sample / mass of compound) × 100.

  • Empirical Formula (EF): the simplest whole-number mole ratio of elements in a compound.

    • EF may differ from the molecular formula; the molecular formula is a whole-number multiple of the EF.

    • Example concept: hydrogen peroxide has EF HO (empirical formula is HO; molecular formula is H₂O₂).

  • Determining EF from percent composition:
    1) Assume a 100 g sample; convert each element’s mass to moles by dividing by the element’s molar mass.
    2) Divide all mole values by the smallest mole value to obtain the simplest whole-number ratio.
    3) If fractions remain, multiply by the smallest factor that yields whole numbers.

  • Examples (from slides):

    • Sulfur oxide with 40.05% S and 59.95% O yields EF SO₃ (S: 1, O: 3).

    • Propane empirical formula: 81.82% C, 18.18% H → EF ≈ C₃H₈ (multiplier 3).

    • Aspirin: 60.00% C, 4.44% H, 35.56% O → EF ≈ C₉H₈O₄ (multiplied by 4 from C₂.25H₂O₁).

    • A multielement example: 38.67% C, 16.22% H, 45.11% N → EF ≈ C₁H₅N₁.

  • Molecular Formula (MF): MF = (EF) × n, where n is a positive integer such that the MF molar mass matches the experimental molar mass.

    • MF representation: MF = (EF) × n; n is determined as n = M{ ext{mol}} / M{ ext{EF}}.

    • Example: If MF mass is 58 g/mol and EF has M_{ ext{EF}} = 29 g/mol (e.g., C₂H₆), then n = 58 / 29 = 2 → MF = C₄H₁₂ (illustrative; use actual EF mass to compute).

  • Practical approach to MF: calculate EF, determine M_{ ext{EF}}, compare with experimental molar mass to obtain n, and then multiply EF by n to get MF.

Examples: Empirical vs Molecular Formulas (Worked Illustrations)

  • Example: Determine empirical formula from percentages and then deduce molecular formula.

    • Step 1: Convert % to mass (for 100 g sample).

    • Step 2: Convert masses to moles.

    • Step 3: Normalize to smallest whole-number ratio.

    • Step 4: If necessary, clear fractions by multiplying by a common factor.

  • Example problem results given in slides:

    • Empirical formulas derived from problems such as aspirin, propane, and others.

    • Molecular formula problems show how to scale EF to MF using molar masses or given molecular weights.

  • Practice problems (sample answers listed):

    • Problem 1: Molecular formula for a compound from % C, % H, % O and molar mass 110.0 g/mol → MF determined.

    • Problem 2: Compound with 49.98 g C and 10.47 g H; molar mass 58.12 g/mol → MF determined.

    • Problem 3: 46.68% N and 53.32% O; molar mass 60.01 g/mol → MF determined; given answer: N₂O₂

Spectroscopic Methods in Organic Analysis

  • Infrared (IR) spectroscopy: identifies presence of functional groups by vibrational transitions; IR absorptions occur at characteristic wavenumbers (cm⁻¹); vibrations are quantized.

  • Ultraviolet (UV) spectroscopy: probes conjugation and pi-bonds; conjugated systems absorb UV/visible light; longer-wavelength absorptions correspond to more extended pi-systems; example shows absorption maxima around 222 nm for isoprene-type structures.

  • Nuclear Magnetic Resonance (NMR) spectroscopy: probes hydrogen (¹H) and carbon (¹³C) environments in molecules; number of signals indicates the number of distinct proton environments; chemical shifts reflect electronic environment; spectrum axes: X = absorption frequency, Y = chemical shift (ppm); reference typically tetramethylsilane (TMS) at 0 ppm.

  • Mass Spectrometry (MS): determines molecular weight and molecular formula; involves ionization to generate molecular ions and fragments; mass-to-charge ratio (m/z) is plotted vs abundance; the tallest peak is the base peak; used to infer fragmentation patterns and molecular weight.

Electromagnetic Radiation and Applications in Spectroscopy

  • Electromagnetic radiation consists of waves with different wavelengths traveling at the speed of light; visible spectrum is part of this; shorter wavelengths carry higher energy.

  • Speed of light in vacuum: c = 3.0 imes 10^8 ext{ m s}^{-1}; energy relates to frequency and wavelength: E = h
    u = rac{hc}{\lambda} where h is Planck’s constant.

  • Shorter wavelengths ⇒ higher energy photons; this underpins absorption in IR, UV, and visible regions and is crucial for interpreting spectroscopic data.

Practical Notes and References

  • The presented content is from a General Chemistry II/Organic Chemistry course at the University of Ghana; it covers foundational topics in organic chemistry, purification, separation techniques, qualitative and quantitative analysis, and instrumental methods.

  • The material emphasizes connections between structure, properties, and analytical techniques, with practical lab considerations (e.g., use of vacuum for mp, boiling chips, TLC visualization, Soxhlet extraction).

  • Several real-world relevance notes are included:

    • Ethylene as a ripening agent in fruits.

    • Formic acid as a simple organic acid present in stings.

    • Purification and characterization techniques essential for isolating and identifying organic compounds in research and industry.

Key Formulas and Quick References

  • Bonding geometry around carbon: ext{sp}^3
    ightarrow ext{tetrahedral} (109.5^ ext{o}); ext{sp}^2
    ightarrow ext{trigonal planar} (120^ ext{o}); ext{sp}
    ightarrow ext{linear} (180^ ext{o})

  • TLC retention factor: ext{Rf} = rac{d{ ext{spot}}}{d{ ext{solvent}}}

  • Distillation purposes: separates components by different boiling points; safe heating relies on boiling chips; simple vs fractional vs vacuum vs steam distillation

  • Purification purification decisions depend on: nature of compound, physical state, quantity, impurity content, stability

  • Lassaigne test indicators (qualitative): Na fusion converts elements to detectable inorganic forms; halides yield AgX precipitates; sulfur/nitrogen tests via Pb(II) and ferric reagents

  • Empirical formula calculation steps:

    • Assume 100 g sample; convert to moles; divide by smallest; adjust to whole numbers; EF is the simplest mole ratio.

  • Molecular formula relation:

    • MF = (EF) × n, where n = rac{M{ ext{mol}}}{M{ ext{EF}}}

  • Empirical vs Molecular Formula relationship:

    • EF may be multiplied by an integer to match the actual molar mass.

  • Percent composition to EF conversion:

    • If 100 g sample, masses in grams equal percent values; convert to moles and normalize as above.

  • Column, GC, and HPLC overview:

    • Column chromatography separates large quantities; GC separates volatile mixtures with a carrier gas; HPLC uses high pressure for finer separation of small, often temperature-sensitive compounds.

Note on Equations and Notation

  • All mathematical expressions are presented in LaTeX format within double dollar signs, e.g., ext{Rf} = rac{d{ ext{spot}}}{d{ ext{solvent}}} and c = rac{E}{ ext{h}
    u} = rac{E}{h} where applicable.

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